1. Field of the Invention
The present invention relates to a radiation image detector that includes a radiation image recording medium capable of recording a radiation image carried by radiation when exposed thereto and detects signals outputted from the medium in accordance with the radiation image recorded thereon. More specifically, it relates to a radiation image detector for detecting doses of radiation while the radiation image recording described above is being performed.
2. Description of the Related Art
A radiation image detector that includes a radiation image recording medium capable of recording a radiation image by storing electric charges into the storage section of the medium in accordance with the dose of radiation such as X-ray transmitted through-a subject and detects signals outputted from the medium in accordance with the radiation image recorded thereon is widely used including, for example, in obtaining medical radiation images, and various types of detectors are proposed.
One such detector is proposed as described, for example, in Japanese Unexamined Patent Publication No. 2000-284056. The radiation image detector described above has a radiation image recording medium comprising a set of layers layered in the order of a first electrode layer which is transparent to radiation; a recording photoconductive layer that generates charges when exposed to radiation; a charge transport layer that acts as substantially an insulator against the charges of latent image and as substantially a conductor for the transport charges having the opposite polarity to that of the charges of latent image; a reading photoconductive layer that generates electric charges when exposed to reading light; and a second electrode layer composed of first wire electrodes which are transparent to reading light and extending linearly, and second wire electrodes which are opaque to reading light and extending linearly, the first and second wire electrodes being disposed alternately in parallel with each other. In the radiation image detector described above, the radiation is emitted on the detector from the side of the first electrode, and the radiation image is recorded by storing electric charges of the latent image in accordance with the dose of radiation emitted thereon at the interface between the recording photoconductive layer and charge transport layer. Thereafter, the radiation image stored in the detector is read out as electrical signals by emitting reading light from the side of the second electrode and detecting the currents generated in the detector, that is, when the reading light is emitted on the detector, it passes through the first wire electrodes and illuminates the reading photoconductive layer generating charge pairs therein, and either negative or positive charges of the charge pairs couple with the charges of the latent image stored in the storage section, while the charges having the other polarity are detected by a current detecting amplifier connected to the first or second wire electrode, thereby the radiation image stored in the detector is read out as electrical signals.
Here, when obtaining a radiation image using the radiation image detector described above, the dose of radiation is controlled in accordance with the region of the human body to be imaged, the type of radiation source used, and the like, thereby the dose of radiation is controlled to an amount which is not harmful to human body, yet sufficient to obtain a radiation image of appropriate quality for diagnosis. In order to implement such dose control described above, the phototimer has conventionally been used. The phototimer is disposed between the radiation image detector and the subject to be imaged so that a gap is left therebetween. This gap causes the radiation image obtained by the radiation image detector to be enlarged, so that the size of the subject on the radiation image may differ from the actual size of the subject. This discrepancy in the size is undesirable in obtaining ordinary radiation images for medical diagnosis and making treatment plans using the radiation image other than the case in which the radiation image is enlarged intentionally. Further, the use of such radiation image for the geometrical measurement of the subject may cause inaccurate results. Still further, the phototimer is a device that compares the dose of radiation detected with a predetermined threshold level, and outputs a signal for terminating the radiation when the detected dose of radiation exceeds the threshold level. It has, however, different sensitivities to different radiation sources and their quality so that the threshold level needs to be adjusted according to the type of radiation source and its quality.
In this connection, certain methods for obtaining the signal for controlling the dose of radiation without using the aforementioned phototimer are proposed as described, for example, in U.S. Pat. No. 6,243,441 and U.S. Patent Laid-Open No. 20010806187. For example, U.S. Pat. No. 6,243,441 proposes a method for obtaining the dose control signal described above for the radiation image detector of so-called a direct conversion type that converts radiation image carried by radiation to electric charges using a photoconductive layer made of, for example, amorphous Selenium (a-Se) for detecting the radiation image, in which microplates, each corresponding to each pixel, for collecting the charges produced in the manner described above are divided into a real-time signal electrode for reading out the real-time signal corresponding to the charges generated in the photoconductive layer while it is being exposed to the radiation and a radiation image signal electrode for reading out the accumulated radiation image signal after the radiation image recording is completed, and the dose control signal for controlling the dose of radiation is outputted based on the real-time signal read out from the real-time signal electrode. U.S. Patent Laid-Open No. 20010806187 proposes a method for obtaining the dose control signal for the radiation image detector of direct conversion type as in the case described above, in which an electrostatic sensing electrode is provided separately, and the dose control signal is outputted based on the real-time signal detected by the electrostatic sensing electrode.
However, in the method in which the microplates provided on a pixel-by-pixel basis are divided as described in the U.S. Pat. No. 6,243,441, the structure of the detector becomes complicated and requires a greater number of interconnections, which requires a complicated manufacturing process and a higher manufacturing cost. Further, it has a switching circuit for switching the divided electrodes so that the image quality may be degraded due to the switching noise. The method described in U.S. Patent Laid-Open No. 20010806187 requires an electrostatic sensing electrode separately as described above, so that an additional cost for that is required. Further, the electrode is placed on the entrance side of the radiation so that the image of the electrode may easily be added to the radiation image. Mammographic images, in particular, are affected greatly by this added image of the electrode since they are usually obtained with a small dose of radiation.